Solder mask is generally formed on a conductor circuit, in order to keep insulation between multiple conductor circuits on the printed circuit board, to protect the conductor circuits from peeling off or oxidation or to prevent unnecessary adhesion of solder to the conductor circuits during soldering work, etc.
As the method to form the solder mask, screen printing method has been generally practiced. The screen printing method, however, is inherently of low resolution power and easy to bring about some defects, such as thin spot, pinhole, breeding and blur. In the case of high density printed circuit boards which have been used in recent years, it is necessary to form the solder mask with high precision and therefore the conventional screen printing method is not satisfactory.
A method has been proposed wherein a photosensitive thermosetting resin composition is coated on a circuit board and a desired pattern is then formed by photographic method, followed by curing by heat to form a cured pattern which is used as the solder mask (see Japanese Kokai Publications Sho 63-286841, Hei 2-28651 and Hei 2 -77749).
In the conventional technology to form the solder mask by the photographic method, a photosensitive thermosetting resin composition is used for forming the solder mask and is therefore prepared from material having both photocurable functional group and thermocurable functional group in the same molecule. In the case of the photosensitive resin composition comprising such materials, number of thermocurable functional groups contained in the resin composition is smaller than the other thermosetting resin compositions which only has thermocurable functional groups. Thus, the thermosetting resin composition used in the photographic method has low heat curing degree and is inferior in performances as solder mask, such as electric insulating property, heat resistance and chemical resistance and the like.
In recent years, it is desired that electrical contacting points are gold-plated in order to reduce electrical resistance. If the gold plating is conducted after forming the solder mask, the solder mask is required to have chemical resistance sufficient to withstand the gold-plating liquid. However, the solder mask formed by the conventional photographic method does not have sufficient chemical resistance to gold plating.